It's not always wise to use your frequent flier miles to buy plane tickets. Sometimes you might be better off holding onto the miles and paying for the ticket or trading the miles for cash. Here's a simple calculation you should to do figure out what to do with your miles.

What's a mile worth? About 1.4 cents, according to SmarterTravel. That's based on most airline programs' 25,000 miles "price" for a coach ticket and an average $350 value for that ticket.

Using this 1.4-cents-per-mile baseline, you can calculate whether any ticket is worth using your miles for. Divide the price of the ticket by the number of miles a ticket can be purchased for. If the per-mile value is over 1.4 cents (or 1.2 cents, if you want to add a hassle cost), then use your miles. SmarterTravel has these examples:

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Cashing in 25,000 miles for a ticket that could be purchased for $100 yields just .4 cents (four tenths of a cent).

On the other hand, redeeming 100,000 miles for a business-class ticket to Europe priced at almost $11,000 yields a nominal per-mile value of 11 cents, slightly less with the hassle factor adjustment.

The math is the same to see if buying merchandise with miles or cashing in your miles (e.g., for a gift card or debit card) would be worth it. For easy comparisons between cashing out or flying, visit Humble Savers' online spreadsheet and plug in your numbers.

Remember too that using frequent flier miles for upgrades instead of free tickets may be a better idea.